The high fatigue resistance of thermoplastic fibers can increase the wear life of garments made primarily of cotton and it is therefore highly desireable to include them in flame resistant cotton fabrics as is described in U.S. Pat. No. 4,920,000. However, because the mechanical durability is significantly enhanced and the fabric contains flammable thermoplastic fibers, garments can lose their flame resistance before they wear out.
Conventional single step flame-retardant processes used for cotton fabrics are not commercially viable for cotton/thermoplastic fabrics because the high level of flame retardant chemicals (5%) normally needed to compensate for the presence of the thermoplastic fibers are deposited preferentially on the surface of yarns creating a crust which causes the fabric to be stiff and uncomfortable.
Conventional single step processes for cotton/synthetic fiber blends also do not produce fabrics with flame-retardant treatment which lasts the life of the garment because the flame retardant readily washes off.
Commercially viable flame resistant cotton/thermoplastic fabrics have been produced through a two treatment process wherein the cotton and the thermoplastic fibers are treated separately using two different flame retardants. For example in U.S. Pat. No. 4,732,789 two different chemical treatments are needed to achieve flame resistance in cotton blends containing thermoplastic fibers.